The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and member of the bHLH/PAS (basic Helix-Loop-Helix/Per-ARNT-SiM) family of chemosensors and developmental regulators. The AhR modulates a variety of cellular responses including drug metabolism, cell proliferation, differentiation, and endocrine signaling. There is a large amount of untended findings that strongly indicate that AhR can be modulated to yield biological responses that can be exploited for the treatment of cancer. Specifically, it is hypothesized that AhR ligands are capable of initiating discrete biological responses and that specific AhR-dependent effects can be utilized for the development of new cancer treatments. During a screen for AhR ligands in our lab, a clinically used drug was identified to enhance AhR-dependent transcription. I have carefully characterized this drug as an AhR ligand, and probed its biological effects. The goal of this research is to demonstrate that this drug functions as an anti-cancer agent via the AhR. The proposed research encompasses three specific aims: Aim 1: Determine the AhR dependency for AhR ligand-induced anti-proliferative effects in human cancer cell lines. AhR expression will be suppressed by AhR antisense oligonucleotides and/or shRNA to determine the requirement of AhR for growth inhibition in cancer cells Aim 2: Determine the structure activity relationship for ligand in activating AhR-dependent transcription and inhibition of cancer cell proliferation. I will use 14 structural analogs of the AhR ligand to determine the structural requirements necessary for induction of AhR-mediated transcription and AhR-dependent inhibition of proliferation Aim 3: Discover how the AhR mediates the anti-proliferative effects of the AhR ligand. I will determine the critical downstream target genes of AhR that mediate the anti-proliferative effects. I propose to identify AhR regulated genes to discover the biological pathways that are perturbed through AhR ligand induced activation. Relevance: Clinical use of this drug in the treatment of cancers has been based primarily on the expression of another hormonal receptor, not the AhR. This research will demonstrate the utility of AhR as a new therapeutic target in the treatment of cancer, and provide immediate impacts in the clinic by broadening this drug's usage to tumors expressing the AhR.